An important access route to organic disulphides consists of the oxidation of mercaptans with sulphur in the presence of a catalyst.
When a mercaptan (RSH) and sulphur are brought into contact with a catalyst, the corresponding disulphide is obtained according to the following reaction scheme: ##STR1## using one sulphur atom S per two molecules of mercaptan.
The formation of the disulphide R--SS--R is generally accompanied by the formation of secondary products, namely polysulphides of analogous structure to the disulphide, but containing a larger number of combined sulphur atoms (R--S.sub.a --R with n&gt;2).
The reactions which lead to the polysulphides can be denoted by the following equations: ##STR2##
It is known that polysulphides (R--S.sub.a --R) can be converted into disulphides (R--SS--R) by reaction with the mercaptan RSH. These reactions may be denoted as follows: ##STR3##
In the case where the polysulphides are completely converted to disulphides, the general equation for this conversion can be outlined as follows: ##STR4##
This reaction of conversion of a polysulphide to disulphide has been exploited in U.S. Pat. No. 3,299,146 in a process for the preparation of dimethyl disulphide, which consists in reacting methyl mercaptan with dimethyl trisulphide: EQU CH.sub.3 --SH.sub.3 --CH.sub.3 +2 CH.sub.3 SH.fwdarw.2 CH.sub.3 --SS--CH.sub.3 +H.sub.2 S
The processes for continuous manufacture of dialkyl disulphides from alkyl mercaptans and sulphur, which are described in European Patent No. 202,420 and European Patent No. 337,839 make use of a synthesis reactor in which the mercaptan and sulphur are introduced. The polysulphides formed in the reaction of oxidation of the mercaptan with sulphur are separated from the disulphides by distillation, then they are recycled to the synthesis reactor to be converted to disulphide. This continuous recycling results in increasing the amount of polysulphides in the reactor and in decreasing the disulphide yield, as well as making needful a distillation column with sizes and characteristics adapted to reaction product compositions rich in polysulphides. For the manufacture of dimethyl disulphide from methyl mercaptan and sulphur, the examples of the above-cited cited European patents show readily that the synthesis reactor produces rather large amounts of dimethyl polysulphides.